1. Field of the Invention
The present invention relates to visual display systems and, in particular, to circuitry for controlling brightness and tint in a RGB monitor on a window-by-window basis. The invention enables the display of motion video and text on the same screen with different brightness and/or tint.
2. Discussion of the Prior Art
A computer display uses a red-green-blue (RGB) monitor that is optimized for clarity so that small text fonts on the screen are easy to read. Distinct colors are few and gradual shading is not needed.
Motion video signals are typically encoded as YCrCb (Luminance, Chrominance-red, Chrominance-blue) so that greater bandwidth can be dedicated to brightness. In a typical video image, brightness will usually vary much more rapidly than color. Far more colors are used to provide realistic shading and variations in color.
When an ordinary television screen is used for computer display, the contrast is typically too high, reducing sharpness and making text difficult to read. Conversely, when digitized video is displayed on a RGB monitor, the image appears dull because brightness and contrast are lower than on a television screen.
Typically, three video digital-to-analog converters (DACs) are used to drive the display of multiple windows on a RGB monitor. A video DAC may consist of several elements which must be carefully controlled to maintain constant analog output for a given digital input code. The digital input code may consist of several bits which change in synchronism. For example, an eight-bit video DAC will receive an eight-bit digital input code to control 255 identical current sources or other devices with commonly-connected outputs. The digital input code determines which and how many of the current sources are enabled. That is, the output of the video DAC will vary from zero current, with none of the current sources enabled, to full scale, a current equal to 255 times the output current of one source, when all current sources are enabled. This output may vary incrementally by the current of one source for the 256 possible values of the binary eight-bit digital input code. The output of the video DAC is resistively terminated to a common ground node and connected to one of the three (RGB) inputs to the monitor. Identical DACs drive the other two monitor inputs.
In a RGB monitor, brightness is controlled by applying a DC offset equally to all three video DAC analog output lines. Tint variation is obtained by applying a DC offset unequally to the three lines. Previously, no variation has been used in the video DAC outputs to provide brightness and tint level variation on a window-to-window basis. The overall screen has been set to optimize for one window of interest. However, in a "windowed" environment, desktop graphics windows require optimization for brightness and clarity that is different from that needed for video windows. Thus, some windows have suffered if the brightness and tint levels were inappropriate for those windows.
Therefore, it would be highly desirable to have available circuitry for controlling the brightness and tint of a display on a window-by-window basis.